Various mechanisms have been proposed as devices for changing the lift characteristics of intake and exhaust valves of an internal combustion engine. U.S. Pat. No. 6,302,070B1 and JP2011-185092A disclose a valve rest mechanism in which a valve lifter is interposed between each valve and the corresponding valve actuation cam, and a slide pin configured to be hydraulically and selectively actuated is incorporated in the valve lifter.
In such a valve rest mechanism, the valve lifter internally defines a slide hole extending in a diametric direction, and a slide pin is slidably received in the slide hole. The slide pin can be axially actuated by hydraulic pressure applied to one of the axial end of the slide pin, and a return spring is provided on the other axial end of the slide pin. The lower side of the slide pin is formed with a flat abutting surface, and a receiving hole is formed in a part of the abutting surface. By controlling the hydraulic pressure supplied to the one end of the slide pin, the slide pin can move between a valve operating position where the stem end of the engine valve abuts the abutting surface of the slide pin, and a valve rest position where the stem end of the engine valve is received in the receiving hole.
In the valve rest mechanism disclosed in U.S. Pat. No. 6,302,070B1, when the hydraulic pressure is not applied to the one end of the slide pin (at low pressure), the stem end of the valve stem is received in the receiving hole of the slide pin so that the engine valve does not open as the valve lifter moves downward. When the hydraulic pressure is supplied to the one end of the slide pin, the stem end of the valve stem comes into contact with the abutting surface of the slide pin as the valve lifter moves downward so that the engine valve opens. In the valve rest mechanism disclosed in JP2011-185092A, when the hydraulic pressure is not applied to the one end of the slide pin (at low pressure), the stem end of the valve stem comes into contact with the abutting surface of the slide pin as the valve lifter moves downward so that the engine valve opens. When the hydraulic pressure is supplied to the one end of the slide pin, the stem end of the valve stem is received in the receiving hole of the slide pin so that the engine valve does not open as the valve lifter moves downward.
Such valve rest mechanisms are typically applied to multi-cylinder engines. In such a case, the hydraulic pressure for the valve rest mechanisms of the different cylinders are not individually controlled, but is commonly controlled. Therefore, when the hydraulic pressure is switched over between a valve operating state and the valve rest state, some of the valve lifters may be in the process of moving upward or downward.
When the hydraulic pressure for one of the valve lifters is switched while the valve lifter is moving upward or downward, and the valve lifter is therefore placed under a compressive load, the slide pin may not move in a stable manner. It is possible that the engine valve may abruptly close or the valve stem could be wedged in the receiving hole. In either case, the durability of the relevant component parts may be adversely affected, and/or undesired noises may be produced.